[MIS07-P03] H2-rich hydrothermal environment in the Enceladus’ subsurface ocean
The results show that the H2 concentration in the fluid increased up to 14.6 mmol/kg 5,760 hours after the beginning of the experiment, which indicates that water-chondrite reactions likely generate H2-rich hydrothermal fluid. 3D tidal heating simulations indicates that more than 10 GW of heat can be generated by tidal friction and that water transport in the tidally heated permeable core can also generate hotspots (1–5 GW) at the seafloor in the south pole region (Choblet et al., 2017). If it is assumed that the heat flux of hotspots is totally carried by 200-degree C hydrothermal venting, the H2 flux at the south pole seafloor is estimated to be 5.2×10^8–2.6×10^9 mol/year, corresponding to 10–260 % of the H2 flux (1–5×10^9 mol/year) observed by Cassini spacecraft. Therefore, the observed H2 flux can be partially or completely explained by hydrothermal activities near the south pole hotspots. Of course, if lower-temperature but global hydrothermal activities are present, the seafloor water-chondrite reactions would sufficiently account for the observed H2 flux.
The high H2 concentration of fluid in the experiment corresponds to those of peridotite-hosted seafloor hydrothermal systems on Earth. Considering that hydrogenotrophic methanogens certainly exist in most of the terrestrial peridotite-hosted hydrothermal systems, the results suggest that the hydrothermal systems on Enceladus are capable of sustaining hydrogenotrophic methanogenesis if temperature of venting hydrothermal fluids is close to 200 degrees C. Further water-chondrite experiments at temperatures lower than 200 degrees C would provide more constraints on the temperature of hydrothermal systems and the habitability of H2-utilizing living forms within Enceladus.